Steel coil springs in the wheel assemblies of automobiles and other vehicles are subjected to very corrosive conditions. Conventional steel springs were expected to rust, and to allow for such rusting, conventional coil springs were formed of appropriately thick steel.
To improve the steering and ride control of automobiles, it is desirable to move the wheels outward, toward the corners of the vehicle. This increases torsional stress on the automobile body structure which must be nullified using a stronger frame assembly or weight reduction of the suspension components moved toward the corners. Reduced diameter, high tensile steel, coil springs weigh less than conventional automotive suspension coil springs, so they offer means to reduce the weight of these components.
High tensile steel, coil springs are scratch and notch sensitive, so they require protection from impact damage caused by flying stones and gravel encountered during driving on paved or unpaved roads. Also, to maintain desired metallurgical properties and prevent premature flex damage failure, the high tensile steel cannot be heated beyond 325.degree. F.
Conventional "E" coat primers and/or epoxy powder coatings used on strut type automotive suspension springs are unacceptable at inhibiting flying stone damage as determined by low temperature gravelometer testing followed by accelerated scab corrosion testing of high tensile springs. Neither layer applied alone will provide the chip and corrosion protection required.
It is an object of the present invention to provide a coating for high tensile steel springs and the like which protects the steel when subject to both impact and corrosive environments. A further requirement when coating high-tensile steel is that the coating be applied, fused, cured, etc. at relatively low temperatures, i.e., 325.degree. F. (163.degree. C.) or below and preferably 300.degree. F. (149.degree. C.) or below. If subjected to significantly higher temperature during coating, the metallurgy of the high-tensile steel tends to change, resulting in flex damage failure of the spring.